JP7715972B2 - pneumatic tires - Google Patents

Description

The present invention relates to a pneumatic tire.

Patent Document 1 is known as a technology for improving uneven wear resistance in pneumatic tires. In Patent Document 1, adjacent blocks on either side of a main groove are offset from each other circumferentially in the tire direction so that the rate of variation in the total contact length at the contact leading edge is 25% or less over one circumference of the tire.

Japanese Patent Application Laid-Open No. 2000-255216

However, Patent Document 1 does not take wet braking performance into consideration, and there is room for improvement in achieving both uneven wear resistance and wet braking performance in pneumatic tires.

The present invention was made in light of the above, and its purpose is to provide a pneumatic tire that can achieve both uneven wear resistance and wet braking performance.

In order to solve the above-mentioned problems and achieve the object, a pneumatic tire according to one aspect of the present invention comprises a carcass layer constituting a framework of the tire, a belt layer disposed on an outer periphery of the carcass layer, and a tread portion provided on the outer side of the belt layer, wherein the tread portion comprises at least two circumferential grooves extending in the tire circumferential direction, shoulder land portions extending in the tire circumferential direction and including tire ground contact edges, and auxiliary grooves which extend in the tire width direction without crossing the tire equatorial plane and which do not extend to the outer side of the shoulder land portions in the tire width direction, excluding the circumferential groove portions, and the auxiliary grooves extend along their entire length. and the secondary groove is inclined with respect to the tire circumferential direction, the secondary groove is arranged so as to intersect with the circumferential main groove, the secondary groove has a short portion on the outer side in the tire width direction and a long portion on the inner side in the tire width direction, with the circumferential main groove as the boundary, the groove center line of the short portion and the groove center line of the long portion are on the same straight line, the direction of inclination of the secondary groove with respect to the tire circumferential direction is the same on both sides of the circumferential main groove that intersects with the secondary groove, the load load rate is set to 70% of the load capacity by air pressure specified in a predetermined standard, and when the road plate is moved while the tire is pressed against the road plate, the circumferential variation rate of the actual contact area image is 5.0% or less.

If the length along the tire circumferential direction passing through the center of the total contact area obtained from the tire contact shape is defined as the central contact length, and the length along the tire width direction passing through the center of the total contact area is defined as the central contact width, it is preferable that the circumferential variation rate of the central contact length is equal to or greater than the circumferential variation rate of the central contact width.

The circumferential variation rate of the central contact length and the circumferential variation rate of the central contact width are preferably defined by the following formulas.
Circumferential variation rate of center contact length≦-3.3×A+8.0
Circumferential variation rate of center contact width≦-1.9×A+3.9
Load factor A = (measured load) / (load capacity by air pressure specified in the standard)

It is preferable that the sub-grooves, each having a groove width of 2 mm or less, are provided at predetermined intervals in the tire circumferential direction in land portions adjacent to the circumferential groove.

If a shoulder lug groove is defined as a groove that extends in the tire width direction and on the outer side of the shoulder land portion in the tire width direction, the width of the shoulder lug groove is preferably 2 mm or less.

It is preferable that the secondary grooves and the shoulder lug grooves are arranged alternately in the tire circumferential direction.

It is preferable that the sub-groove extends across the circumferential groove, and the angle of the extension direction of the sub-groove with respect to the tire width direction is equal to or greater than 0 degrees and equal to or less than 45 degrees.

The tread portion preferably has a tread pattern that repeats at a predetermined pitch, and the pitch ratio of the predetermined pitch is preferably 1.38, 1.19, or 1.00.

The circumferential variation rate of the actual contact area image is preferably defined by the following formula:
Circumferential fluctuation rate of actual contact area image≦−3.4×A+7.38
Load factor A = (measured load) / (load capacity by air pressure specified in the standard)

The pneumatic tire of the present invention is able to achieve both uneven wear resistance and wet braking performance.

FIG. 1 is a meridian cross-sectional view showing a pneumatic tire according to an embodiment of the present invention. FIG. 2 is a plan view showing a tread portion of the pneumatic tire shown in FIG. FIG. 3 is a diagram showing the circumferential average and circumferential variation rate of the actual contact area image of a tire. FIG. 4 is a schematic diagram showing the configuration of a tire contact patch shape analysis device used to analyze the tire contact patch shape. FIG. 5 is a block diagram showing the functions of the tire contact shape analysis device shown in FIG. FIG. 6 is a flow chart showing an example of the operation of the tire contact shape analysis device. FIG. 7 is a flowchart showing an example of the calculation process in FIG. FIG. 8 is a diagram showing an example of a specific arrangement of the camera and the illumination lamp when acquiring a ground surface image. FIG. 9 is a diagram showing an example of a specific arrangement of the camera and the illumination lamp when acquiring a ground surface image. FIG. 10 is a diagram showing an example of a specific arrangement of the camera and the illumination lamp when acquiring a ground surface image. FIG. 11 is a view of the side surface of the tire as viewed from the direction of the rotation axis. FIG. 12 is a view of the tire viewed from below the lower surface side of the road plate. FIG. 13 is a view of the side of the tire viewed from a direction perpendicular to the rotation axis. FIG. 14 is a view of the side surface of the tire as viewed from the direction of the rotation axis. FIG. 15 is a view of the tire viewed from below the lower surface side of the road plate. FIG. 16 is a view of the side of the tire viewed from a direction perpendicular to the rotation axis. FIG. 17 is a diagram showing an example of an image obtained by photographing. FIG. 18 is a diagram illustrating an example of a GCA. FIG. 19 is an explanatory diagram of the expansion process. FIG. 20 is an explanatory diagram of the contraction process. FIG. 21 is a diagram illustrating an example of a GCA. FIG. 22 is a diagram showing another example of a tread portion of a pneumatic tire. FIG. 23 is a diagram showing another example of a tread portion of a pneumatic tire. FIG. 24 is a diagram showing examples of ACA circumferential variation rates for various tires. FIG. 25 is a table showing the tires that were the subject of the analysis. FIG. 26 is a diagram showing examples of the circumferential variation rate of the center contact length for various tires. FIG. 27 is a diagram showing examples of the circumferential variation rate of the center contact width for various tires.

Embodiments of the present invention are described in detail below with reference to the drawings. In the following description of each embodiment, components that are the same or equivalent to those in other embodiments are given the same reference numerals, and their description will be simplified or omitted. The present invention is not limited to each embodiment. Furthermore, the components of each embodiment include those that are easily replaceable by those skilled in the art, or those that are substantially identical. The configurations described below can be combined as appropriate. Furthermore, configurations can be omitted, replaced, or modified within the scope of the gist of the invention.

Figure 1 is a meridian cross-sectional view showing a pneumatic tire according to an embodiment of the present invention. Figure 1 shows a cross-sectional view of one side region in the tire radial direction. Figure 1 also shows a radial tire for passenger cars as an example of a pneumatic tire 1 (hereinafter referred to as tire 1, as appropriate). Figure 2 is a plan view showing the tread portion 3 of the pneumatic tire 1 shown in Figure 1.

In the following description, a meridian section of a tire refers to a cross section of the tire cut by a plane including the tire's rotation axis (not shown). The tire radial direction refers to the direction perpendicular to the tire's rotation axis (not shown), the tire radial inner side refers to the side toward the rotation axis in the tire radial direction, and the tire radial outer side refers to the side away from the rotation axis in the tire radial direction. The tire circumferential direction refers to the direction around the rotation axis as the central axis. The tire width direction refers to the direction parallel to the rotation axis, the tire width inner side refers to the side toward the tire equatorial plane (tire equator line) CL in the tire width direction, and the tire width outer side refers to the side away from the tire equatorial plane CL in the tire width direction. The tire equatorial plane CL is a plane perpendicular to the tire's rotation axis and passing through the center of the tire width of the tire 1. The tire equator line refers to a line on the tire equatorial plane CL that runs along the tire circumferential direction of the tire 1. In this embodiment, the tire equator line is given the same symbol "CL" as the tire equatorial plane.

In Figure 1, points T and T are ground contact edges. The ground contact edges refer to the outermost edges in the tire width direction of the area where the tread portion 3 of the tire 1 comes into contact with the road surface when the tire 1 is mounted on a specified rim, inflated to a specified internal pressure, and subjected to 70% of the specified load. The ground contact edges are continuous in the tire circumferential direction.

Here, the specified rim refers to the "standard rim" specified by JATMA, the "design rim" specified by TRA, or the "measuring rim" specified by ETRTO. Furthermore, the specified internal pressure refers to the "maximum air pressure" specified by JATMA, the maximum value listed in the "tire load limits at various cold inflation pressures" specified by TRA, or the "inflation pressures" specified by ETRTO. For example, the specified internal pressure is 250 kPa.

This pneumatic tire 1 has an annular structure centered on the tire rotation axis, and includes a pair of bead cores 110, 110, a pair of bead fillers 12, 12, a carcass layer 13, a belt layer 14, a tread rubber 2, a pair of sidewall rubbers 160, 160, and a pair of rim cushion rubbers 17, 17. An inner liner 150 is formed along the carcass layer 13 on the inside of the carcass layer 13, or on the inner side of the carcass layer 13 within the pneumatic tire 1.

A pair of bead cores 110, 110 are annular members made up of multiple bundled bead wires, and form the cores of the left and right bead sections 10. A pair of bead fillers 12, 12 are positioned on the outer periphery of the pair of bead cores 110, 110 in the tire radial direction, respectively, to form the bead sections 10.

The carcass layer 13 has a single-layer structure consisting of one carcass ply or a multi-layer structure consisting of multiple carcass plies stacked together, and is toroidally stretched between the left and right bead cores 110, 110 to form the tire's framework. Both ends of the carcass layer 13 are wrapped around and secured to the outside in the tire width direction, enveloping the bead cores 110 and bead fillers 12. The carcass ply of the carcass layer 13 is formed by covering multiple carcass cords made of steel or organic fiber material (e.g., aramid, nylon, polyester, rayon, etc.) with coating rubber and rolling them.

The belt layer 14 is composed of a pair of cross belts 141, 142 and a belt cover 143 laminated together, and is positioned around the outer periphery of the carcass layer 13. The pair of cross belts 141, 142 are constructed by covering multiple belt cords made of steel or organic fiber material with coating rubber and rolling them, and have a cord angle of 15 degrees or more and 55 degrees or less in absolute value. The pair of cross belts 141, 142 also have cord angles (defined as the inclination angle of the belt cords in the longitudinal direction relative to the tire circumferential direction) of opposite signs, and are laminated with the belt cords' longitudinal directions crossing each other (a so-called cross-ply structure).

The belt cover 143 is constructed by covering a belt cover cord made of steel or organic fiber material with coating rubber, and has a cord angle of 0 degrees or more and 10 degrees or less in absolute value. The belt cover 143 is, for example, a strip of material made by covering one or more belt cover cords with coating rubber, and is constructed by wrapping this strip of material spirally around the outer surfaces of the cross belts 141, 142 multiple times in the tire circumferential direction. The belt cover 143 is positioned to cover the entire area of the cross belts 141, 142.

The tread rubber 2 is arranged on the radial outer periphery of the carcass layer 13 and belt layer 14 to form the tread portion 3 of the tire. A pair of sidewall rubbers 160, 160 are arranged on the outer side of the carcass layer 13 in the tire width direction to form left and right sidewall portions 6. A pair of rim cushion rubbers 17, 17 are arranged on the radial inner side of the left and right bead cores 110, 110 and the turned-up portion of the carcass layer 13, respectively, to form the contact surfaces of the left and right bead portions 10 with the rim flange.

[Tread pattern]
As shown in Fig. 2, a plurality of circumferential main grooves 20 are provided in the tread portion 3. In this example, two circumferential main grooves 20 are provided in the tread portion 3. The pneumatic tire 1 of this example has in the tread portion 3 two circumferential main grooves 20 extending in the tire circumferential direction, a center land portion 31 and shoulder land portions 33 partitioned into three rows by the two circumferential main grooves 20, and a plurality of sub-grooves 41.

The center land portion 31 is a land portion between two circumferential main grooves 20. The center land portion 31 includes the tire equatorial plane CL. The center land portion 31 extends continuously in the tire circumferential direction. The shoulder land portions 33 are land portions on the outer side of the circumferential main grooves 20. The land portions within a range of 1/4 of the contact width of the tread portion 3 from the outer side in the tire width direction can also be called shoulder land portions 33. The shoulder land portions 33 extend continuously in the tire circumferential direction.

The circumferential main groove 20 is a circumferential groove that has a wear indicator that indicates the end of wear, and generally has a groove width of 5.0 mm or more and a groove depth of 7.5 mm or more.

The secondary grooves 41 are provided in the center land portion 31 and shoulder land portions 33 adjacent to the circumferential main groove 20. The secondary grooves 41 extend in the tire width direction, penetrating the circumferential main groove 20. One end of each secondary groove 41 terminates in the center land portion 31. The other end of each secondary groove 41 terminates in the shoulder land portion 33. The secondary grooves 41 are provided at predetermined intervals in the tire circumferential direction. In this example, the secondary grooves 41 do not intersect with each other. In this example, the secondary grooves 41 are provided parallel to each other. In this example, each secondary groove 41 has a long portion 41a provided in the center land portion 31 and a short portion 41b provided in the shoulder land portion 33.

In Figure 2, the symbol T indicates the tire ground contact edge. The tire ground contact edge T refers to the edge in the tire width direction of the portion of the tread portion 3 that comes into contact with the ground when the tire 1 is mounted on a standard rim, inflated to the standard internal pressure, placed vertically on a flat surface, and under a standard load.

Groove width is measured as the maximum distance between the left and right groove walls at the groove opening when the tire 1 is mounted on a specified rim, inflated to the specified internal pressure, and in an unloaded state. In configurations where the land portion has a notch or chamfered edge, the groove width is measured based on the intersection of the tread surface and an extension of the groove wall in a cross-sectional view normal to the groove length direction. Furthermore, in configurations where the groove extends in a zigzag or wavy pattern around the tire circumference, the groove width is measured based on the center line of the groove wall amplitude.

Groove depth is measured as the maximum distance from the tread surface to the bottom of the groove when tire 1 is mounted on a specified rim, inflated to the specified internal pressure, and under no load. Furthermore, if the groove has partial unevenness or sipes at the bottom of the groove, these are excluded from the measurement of groove depth.

A "regulatory rim" is a rim that is specified for each tire 1 in the standard system that includes the standard on which tire 1 is based. For example, in the case of JATMA, it is a standard rim, in the case of TRA, it is a "Design Rim," and in the case of ETRTO, it is a "Measuring Rim." However, if tire 1 is an original equipment tire, the original wheel on which tire 1 is mounted shall be used.

"Specified internal pressure" refers to the air pressure specified for each tire 1 in the standard system that includes the standard on which tire 1 is based. For JATMA, this is the maximum air pressure; for TRA, this is the maximum value listed in the table "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES"; and for ETRTO, this is the "INFLATION PRESSURE." However, if tire 1 is an original equipment tire, this refers to the air pressure indicated on the vehicle.

"Specified load" refers to the load specified for each tire 1 in the standard system that includes the standard on which tire 1 is based. For JATMA, this is the maximum load capacity. For TRA, this is the maximum value listed in the table "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES." For ETRTO, this is the "LOAD CAPACITY." However, if tire 1 is for a passenger car, this load shall be equivalent to 88% of the above load. If tire 1 is a tire fitted to an original equipment vehicle, this wheel load shall be calculated by dividing the front and rear axle loads listed on the vehicle's inspection certificate by the number of tires.

In Figure 2, the ratio of the tread pattern pitch PT of the tread portion 3, i.e., the pitch ratio, is, for example, 1.38, 1.19, or 1.00. In other words, if the minimum value of the pitch PT is 1.00, it is set to 1.19 or the maximum value of 1.38. By adopting this pitch ratio, uneven wear resistance and wet braking performance are further improved.

[ACA average and fluctuation rate]
Fig. 3 is a diagram showing the circumferential average and circumferential variation rate of the actual contact area image (ACA) of the tire 1. As shown in Fig. 3, the ACA varies depending on the rotation angle of the tire 1. The ACA is the total area of the blocks in the tread that are in contact with the road surface. A total ground contact area image (GCA), which will be described later, is the total area enclosed by the outer ring line when the grooves are filled in the ACA.

The horizontal axis in Figure 3 is the tire circumferential position [deg], and the vertical axis in Figure 3 is the ACA [ ^cm2 ]. In Figure 3, when the load load rate is 70% of the load capacity by air pressure specified in a predetermined standard, it is preferable that the circumferential variation rate of the ACA is 5.0% or less. This can improve uneven wear resistance, i.e., heel-and-toe wear, polygonal wear (wear with a certain percentage of angles in the tire circumferential direction), and wet braking performance.

In FIG. 3, the dashed line SS1 represents the circumferential average Vave of the ACA. The circumferential average Vave is the arithmetic average of the contact characteristics in all regions at positions on the tire circumference. The reciprocating arrow SS2 represents the circumferential fluctuation rate Vfluc of the ACA. The circumferential fluctuation rate Vfluc is the ratio of the difference between the maximum value Vmax and the minimum value Vmin to the circumferential average Vave. In other words, the circumferential fluctuation rate Vfluc can be calculated using the following formula (1):
Vfluc=(Vmax-Vmin)/Vave×100...(1)

Here, the maximum value Vmax is the maximum value of the contact characteristic (here, ACA) over the entire region on the tire circumference. The minimum value Vmin is the minimum value of the contact characteristic (here, ACA) over the entire region on the tire circumference.

The tire size of tire 1 was 195/65R15 91Q, and the ACA average Vave and circumferential fluctuation rate Vfluc were measured under conditions of an air pressure of 230 kPa and a load of 4.5 kN. The results were an ACA average Vave of 138.1 ^cm2 and an ACA circumferential fluctuation rate Vfluc of 4.3%. In other words, the ACA circumferential fluctuation rate of tire 1 was 5.0% or less.

The load factor was set to 70.1% of the JATMA load capacity by air pressure (600 [kg] x gravitational acceleration 9.8 [m/ ^s2 ]). The measurements were performed in two sessions, and after the first measurement was completed, the tire 1 was rotated half a circumference, and the starting position was used to perform the second measurement. Note that Figure 3 shows the measurement results for half a circumference of the tire, as the tire circumferential position was changed from 0 [deg] to 180 [deg] in one measurement. It is preferable to obtain the ACA circumferential variation for the entire circumference of the tire.

[Tire contact shape analysis device]
Hereinafter, a description will be given of a tire contact patch shape analyzer for measuring the circumferential variation rate of the ACA of the tire 1. Fig. 4 is a configuration diagram that schematically shows the tire contact patch shape analyzer 100 used to analyze the contact patch shape of the tire 1. Fig. 5 is a block diagram showing the functions of the tire contact patch shape analyzer 100 shown in Fig. 4. In these figures, Fig. 4 schematically shows the overall configuration of the tire contact patch shape analyzer 100, and Fig. 5 shows the main functions of the tire contact patch shape analyzer 100.

The tire contact patch shape analysis device 100 is applied to a system that analyzes the contact patch 61 of a tire 1 by acquiring an image of the contact patch 61. The tire contact patch shape analysis device 100 includes a tire testing machine 200 and a tire contact patch analysis device 201.

The tire testing machine 200 is a device that applies test conditions to the tire 1 to be analyzed. In the configuration shown in Figure 4, the tire testing machine 200 has a support device 300, a drive device 5, and a road surface plate 11. The support device 300 is a device that rotatably supports the tire 1 and has a rim 400 on which the tire 1 is mounted. The drive device 5 is a device that applies driving force to the tire 1 and road surface plate 11. The drive device 5 is composed of a motor 60 that drives the tire 1 and road surface plate 11, and a motor control device 7 that controls the motor 60.

The drive unit 5 includes gears (not shown) and drives the road surface plate 11 horizontally. The drive unit 5 drives the road surface plate 11 so that it moves relative to the tire 1, which is the subject of analysis.

In this tire testing machine 200, a support device 300 supports a tire 1 mounted on a rim 400, and the tire 1 is pressed against the upper surface 11U, which is one of the main surfaces of the road surface plate 11, to apply a load to the tire 1. In this example, the load is applied by fixing the distance between the road surface plate 11 and the axis of rotation of the tire 1.

The road surface plate 11 recreates a flat road surface. When the tire 1 is pressed against the road surface plate 11, the contact patch 61 deforms in the same way as when the tire 1 is traveling on a flat road surface. By driving the road surface plate 11 horizontally, the rolling state of the tire 1 when the vehicle is traveling is recreated with the surface of the road surface plate 11 as the road surface, allowing dynamic contact characteristics to be analyzed.

The moving speed of the road surface plate 11 is preferably as slow as possible to ensure measurement accuracy. If the moving speed of the road surface plate 11 is made high, the exposure time must be shortened to prevent image blurring when photographing the tire contact surface. This results in an insufficient amount of light from the lighting, reducing the difference in brightness between the contact area and non-contact area, making it difficult to ensure accuracy in the circumferential variation rate of the ACA. For these reasons, it is preferable to make the moving speed of the road surface plate 11 slow.

The support device 300 can also displace the rim 400 to adjust the positional relationship between the tire 1 and the road surface plate 11, thereby imparting a slip angle or tilt angle to the tire 1. The drive device 5 can drive the motor 60 via the motor control device 7 to rotate the rim 400 by a predetermined angle. The support device 300 and drive device 5 can also change test conditions by adjusting the load, rotational speed, slip angle, tilt angle, etc. The predetermined angle is the angle in the tire rotation direction around the tire rotation axis. Here, for example, by using a stepping motor as the motor 60, the rim 400, i.e., the tire 1, can be rotated by an angle corresponding to the number of pulses provided to the motor 60. For example, the tire 1 can be rotated by an angle of 1 degree or less. If the ACA is obtained every degree around the tire circumference, for example, if the tire size is 195, the ACA can be obtained at approximately 6 mm intervals.

The road surface plate 11 is a light-transmitting plate that has the property of transmitting light. The road surface plate 11 does not need to transmit 100% of light; it only needs to have a light transmittance that allows the surface of the tire 1 to be photographed through the road surface plate 11. The road surface plate 11 is, for example, a flat plate made of acrylic resin or a flat glass plate. The contact state between the tire 1 and the flat plate is photographed and analyzed, allowing for a more realistic ground contact state of the tire 1 to be analyzed. The road surface plate 11 is moved by the drive device 5, which is the road surface drive unit, and the ground contact characteristics of the tire 1 can be obtained for each measurement angle when the tire 1 is in contact with the main surface of the road surface plate 11. Note that there are no specified specifications for the road surface plate 11, such as plate thickness or refraction angle.

The tire testing machine 200 also has a camera 15, which is an imaging unit that images the tire 1, and an illumination lamp 16, which is a light source. The camera 15 is, for example, a CCD (Charge Coupled Device) camera. The camera 15 is fixed to the tire testing machine 200. The camera 15 images the tire 1 through the road plate 11, thereby capturing an image of the tire contact surface 61 of the tire 1 pressed against the road plate 11. More specifically, the camera 15 is disposed on the lower surface 11D, the other main surface of the road plate 11, with its optical axis oriented perpendicular to the lower surface 11D, and captures an image of the tire 1 through the road plate 11 from the lower surface 11D side. In this way, the camera 15 captures an image of the tire 1, including at least the contact surface 61, and generates digital image data of the tire 1, including the contact surface 61.

The lighting lamps 16 are lamps that illuminate the shooting range of the camera 15 and are configured, for example, as halogen lamps. The lighting lamps 16 emit light onto the contact surface 61 of the tire 1 that is pressed against the road surface plate 11. The lighting lamps 16 emit light from the lower surface 11D side of the road surface plate 11, through the road surface plate 11, or from between the upper surface 11U side of the road surface plate 11 and the tire 1. The multiple lighting lamps 16 are each arranged in positions other than the position where the road surface plate 11 moves. Note that even if the road surface plate 11 moves, the positions of the tire rotation axis, camera 15, and lighting lamps 16 remain fixed throughout.

The number of these lighting lamps 16 may vary depending on the test conditions in the tire testing machine 200. For example, if the load applied when pressing the tire 1 against the road plate 11 is small, the contact area will be small, so a relatively small number of lighting lamps 16 may be provided, and they may be placed in just two locations diagonally relative to the direction of movement of the road plate 11. In contrast, if the load applied when pressing the tire 1 against the road plate 11 is large, the contact area will be large, so it will be necessary to illuminate the contact surface 61 with light from more directions. For this reason, in this case, the lighting lamps 16 are placed in four or more locations surrounding the contact surface 61. Furthermore, these lighting lamps 16 may be of the constant lighting type or the flashing type.

The tire contact patch analysis device 201 is, for example, a PC (Personal Computer) with a predetermined analysis program installed. It processes images of the tire 1 input from the camera 15 to analyze the tire's contact patch 61. The analysis of the tire's contact patch 61 includes calculating the contact patch 61 based on the captured image of the tire 1. The tire contact patch analysis device 201 includes a processing device 23 that performs calculations such as analyzing the contact patch 61 and storing data; an input unit 21 through which an operator inputs data to the tire contact patch analysis device 201; and a display unit 22 that displays analysis results and various information. The input unit 21 uses a pointing device such as a keyboard or a mouse, and the display unit 22 uses a display device such as a liquid crystal display. The input unit 21 and display unit 22 are electrically connected to the processing device 23, allowing the operator to input data using the input unit 21 while viewing the display unit 22. In addition, the camera 15 is connected to the processing device 23 of the tire contact patch analysis device 201, which enables the tire contact patch analysis device 201 to acquire images captured by the camera 15.

The processing device 23 of the tire contact patch analysis device 201 is configured with a processing unit 23a having a CPU (Central Processing Unit) or the like, and a memory unit 35 such as RAM (Random Access Memory). The processing unit 23a and memory unit 35 configured in this manner may be provided in the same housing, or in different housings, or multiple memory units 35 may be provided in both forms.

The processing unit 23a of the processing device 23 functionally includes a road surface driving unit 231, a ground contact characteristic acquisition unit 232, a ground contact characteristic variation calculation unit 233, and a memory unit 35. The road surface driving unit 231 controls the driving device 5 to rotate the tire 1.

The ground contact characteristic acquisition unit 232 acquires a photographed image of the contact surface 61 of the tire 1 taken by the camera 15. The photographed image is a digital image of the contact surface 61 of the tire 1 taken by the camera 15. The ground contact characteristics that can be acquired by the ground contact characteristic acquisition unit 232 are characteristics that can be acquired based on the image of the contact surface 61 of the tire 1, such as the ACA, GCA, contact length, contact width, and rectangularity.

The ground contact characteristic variation calculation unit 233 calculates the circumferential variation rate of the contact area. In this example, the ground contact characteristic variation calculation unit 233 has an circumferential average value calculation unit 233a and an circumferential variation rate calculation unit 233b. The circumferential average value calculation unit 233a calculates the average value of the ground contact characteristics. The circumferential variation rate calculation unit 233b calculates the variation rate of the ground contact characteristics.

The memory unit 35 pre-stores an analysis program used by the tire contact patch analysis device 201. When acquiring the contact characteristics of the contact patch 61 of the tire 1, the processing unit 23a calls up the program stored in the memory unit 35, and the processing unit 23a executes operations in accordance with the program, thereby performing each function.

The tire contact contour analysis device 100 according to this embodiment is configured as described above. The operation of the tire contact contour analysis device 100 will be described below. When analyzing the contact patch 61 of a tire 1 using the tire contact contour analysis device 100, the tire 1 is attached to the support device 300 of the tire testing machine 200, and the road surface plate 11 is moved while the tire 1 is pressed against it. As a result, the tire 1 is rotated, and the contact patch 61 is photographed by the camera 15 at predetermined rotation angles (measurement angles). During this process, the tire 1 is illuminated from multiple directions by multiple illumination lamps 16. This allows the camera 15 to photograph the tire 1 with a difference in brightness between the contact patch 61 and areas other than the contact patch 61. The captured images are acquired by the tire contact patch analysis device 201, which then analyzes the contact patch 61 based on the acquired images.

[Lighting conditions for shooting]
When acquiring an image of the contact area of the ground contact surface 61 of the tire 1, it is preferable to arrange the lighting lamps 16 on the upper surface 11U side of the road surface plate 11 so as to surround the contact area 61. When acquiring an image of the contact area of the ground contact surface 61 of the tire 1, it is preferable to acquire the image by irradiating the tire 1 with light from the lighting lamps 16 arranged on the upper surface 11U side so as to surround the contact area. An example of the arrangement of the lighting lamps 16 will be described later.

[Operation of tire contact shape analysis device]
6 is a flow diagram showing an example of the operation of the tire contact shape analysis device 100. When analyzing a tire 1, the tire contact shape analysis device 100 irradiates light from the illumination lamp 16 onto the tire 1 pressed against the road surface plate 11 when an operation is performed on the input unit 21 (step S0). Next, the tire contact shape analysis device 100 starts driving the motor 60 using the motor control device 7 (step S1). While continuing to drive the motor 60 (step S2), the tire contact shape analysis device 100 determines whether the rotation angle of the tire 1 has reached a measurement angle (step S3). The measurement angle is the angle at which contact shape data is acquired. If the rotation angle of the tire 1 has not reached the measurement angle, the tire contact shape analysis device 100 continues driving the motor 60 (step S3, No → S2).

When the rotation angle of the tire 1 reaches the measurement angle, the tire contact shape analysis device 100 photographs the tire 1 with the camera 15 and acquires contact characteristic data (step S3, Yes → S4). Furthermore, the tire contact shape analysis device 100 stores the acquired data in the memory unit 35 (step S5).

The tire contact patch shape analysis device 100 determines whether all data acquisition has been completed (step S6). If all data acquisition has not been completed, the tire contact patch shape analysis device 100 continues driving the motor 60 (step S6, No → S2).

When all data acquisition has been completed, the tire contact shape analysis device 100 performs a calculation process for the circumferential average value and circumferential variation rate (Step S6, Yes → S7). The tire contact shape analysis device 100 outputs the calculation results of the circumferential average value and circumferential variation rate to the display unit 22 or the like (Step S8). The tire contact shape analysis device 100 then stops driving the motor 60 (Step S9), and the process ends. Through the above process, contact characteristic data for a dry road surface can be acquired.

Figure 7 is a flow diagram showing an example of the calculation process (step S7) in Figure 6. Figure 7 is a flow diagram showing the process of calculating the circumferential average of the contact characteristics and the circumferential variation rate of the contact characteristics.

In Figure 7, the tire contact patch shape analysis device 100 calculates the circumferential average value Vave of the contact patch characteristics (step S7a). Next, the tire contact patch shape analysis device 100 calculates the circumferential fluctuation rate Vfluc of the contact patch characteristics (step S7b). The circumferential fluctuation rate Vfluc is the ratio of the difference between the maximum and minimum values (Vmax - Vmin) to the circumferential average value Vave, and can be calculated using the above-mentioned formula (1). If the circumferential fluctuation rate Vfluc of the ACA is small, there is a possibility that uneven wear resistance will be improved.

[Sub-groove width and inclination angle]
Referring again to Figure 2, the sub-groove 41 extends in the tire width direction. The groove width of the sub-groove 41 is preferably 2 mm or less. This suppresses circumferential variations in the ACA and the center contact width. Note that if the groove width of the sub-groove 41 exceeds 2 mm, the contact area decreases and uneven wear resistance deteriorates, which is undesirable.

One end of the secondary groove 41 terminates within the center land portion 31, and the other end of the secondary groove 41 terminates within the shoulder land portion 33. The secondary groove 41 extends outward in the tire width direction from the center land portion 31, straddling the circumferential main groove 20, and extending to the shoulder land portion 33. In other words, the secondary groove 41 extends in the tire width direction, penetrating the circumferential main groove 20.

The inclination angle α of the extension direction of the secondary groove 41 relative to the tire width direction is preferably between 0 degrees and 45 degrees. If the inclination angle α is within this range, water discharge efficiency is improved and wet braking performance is further improved. It is not preferable for the inclination angle of the secondary groove 41 relative to the tire width direction to exceed 45 degrees, as this reduces water discharge efficiency and wet braking performance. Note that if the secondary groove 41 is not linear, the inclination angle α of the secondary groove 41 is the inclination of the straight line connecting both ends of the groove centerline.

[Examples of specific layouts and photographed images]
Next, specific examples of the arrangement of the camera 15 and the illumination lamps 16 will be described. FIGS. 8 to 10 are diagrams showing specific examples of the arrangement of the camera 15 and the illumination lamps 16 when acquiring a ground contact patch image. The following describes a case where the illumination lamps 16 include lamps 161 to 168. FIG. 8 is a diagram showing the arrangement of the lamps as viewed from a direction along the rotation axis of the tire 1. FIG. 9 is a diagram showing the arrangement of the lamps as viewed from the upper surface 11U side of the road surface plate 11, which is a transparent plate. FIG. 10 is a diagram showing the arrangement of the lamps as viewed from a direction perpendicular to the rotation axis of the tire 1. In the following description, the direction along the rotation axis of the tire 1 is called the tire width direction, and the direction perpendicular to the rotation axis is called the tire circumferential direction.

[Lighting installed on the upper surface of the road board]
8 to 10 are diagrams showing examples of the arrangement of lamps 161, 162, 163, and 164 provided on the upper surface 11U side of the road surface plate 11. Fig. 8 is a diagram showing the side of the tire 1 as viewed from the rotation axis direction. Fig. 9 is a diagram showing the tire 1 as viewed from above the upper surface 11U side of the road surface plate 11. Fig. 10 is a diagram showing the side of the tire 1 as viewed from a direction perpendicular to the rotation axis.

Referring to Figures 8 to 10, a tire 1 is in contact with the upper surface 11U of the road plate 11. A camera 15 is provided on the lower surface 11D side of the road plate 11. The camera 15 is positioned so that its optical axis 151 is positioned on the normal to the center point of the tire 1 contact patch. By positioning the optical axis 151 of the camera 15 so that it passes through the center point of the contact patch, the contact patch can be photographed from the normal direction of the center point of the contact patch. This ensures stable analysis accuracy. The closer to the edge of the captured image, the greater the influence of lens aberration, causing fluctuations in spatial resolution and unstable analysis accuracy. By positioning the camera 15 in this way, the influence of lens aberration can be minimized.

Referring to Figures 8 to 10, lamps 161, 162, 163, and 164 are arranged on the upper surface 11U side of the road plate 11. Lamps 161, 162, 163, and 164 are arranged at positions spaced apart from the tire 1 in the tire travel direction and tire width direction. Lamps 161 and 162 and lamps 163 and 164 are located on different sides of the tire 1. In this way, lamps 161 to 164 are arranged to surround the contact patch of the tire 1. If the contact patch of the tire 1 is not illuminated on all sides, it becomes difficult to define the outline of the contact shape, which may reduce analysis accuracy. In contrast, by arranging lamps 161 to 164 to surround the contact patch of the tire 1 and illuminating the contact patch on all sides, the outline of the contact shape can be made clearer, improving analysis accuracy.

Here, in Figures 8 and 10, H1 to H4 represent the distances from the center of the light-emitting surface of each lamp 161 to 164 to the upper surface 11U of the road plate 11. In Figure 8, θ1 to θ4 represent the inclination angles of each lamp 161 to 164, i.e., the angles of the light irradiation direction relative to the upper surface 11U of the road plate 11. In Figure 9, D1 to D4 represent the distances in the tire traveling direction from the center of the light-emitting surface of each lamp 161 to 164 to the center of the tire 1. In Figure 9, A1 to A4 represent the distances in the tire width direction from the center of the light-emitting surface of each lamp 161 to 164 to the tire center.

In this example, the distances and angles for each lamp 161-164 were set as follows: distance H1 = 85 mm, distance H2 = 120 mm, distance H3 = 130 mm, distance H4 = 125 mm, distance D1 = 1050 mm, distance D2 = 1020 mm, distance D3 = 980 mm, distance D4 = 1150 mm, distance A1 = 390 mm, distance A2 = 320 mm, distance A3 = 380 mm, distance A4 = 340 mm, angle θ1 = 4.9 [deg], angle θ2 = 5.3 [deg], angle θ3 = 5.3 [deg], angle θ4 = 5.8 [deg].

[Lighting installed on the underside of the road board]
11 to 16 are diagrams showing examples of the arrangement of lamps 165, 166, 167, and 168 provided on the lower surface 11D side of the road surface plate 11. As shown in Figs. 11 to 16, it is preferable to provide lamps 165, 166, 167, and 168 on the lower surface 11D side of the road surface plate 11.

Figure 11 is a view of the side of the tire 1 from the direction of the rotation axis. Figure 12 is a view of the tire 1 from below the lower surface 11D side of the road plate 11. Figure 13 is a view of the side of the tire 1 from a direction perpendicular to the rotation axis.

Referring to Figures 11 to 13, lamps 165 and 166 are arranged on the underside 11D of the road plate 11. Lamps 165 and 166 are arranged at positions spaced apart from each other in the tire width direction relative to the tire 1. Lamps 165 and 166 are located on different sides of the tire 1.

As shown in Figures 11 and 12, lamps 165 and 166 preferably have a length in the tire travel direction and emit light in a direction parallel to the tire width direction. This improves measurement accuracy.

In Figures 12 and 13, H5 and H6 are the distances from the center of the light-emitting surface of each lamp 165, 166 to the underside 11D of the road plate 11. In Figure 13, θ5 and θ6 are the inclination angles of each lamp 165, 166, i.e., the angles of the light irradiation direction relative to the underside 11D of the road plate 11. In Figure 12, D5 and D6 are the distances in the tire width direction from the center of the light-emitting surface of each lamp 165, 166 to the center of the tire 1.

In this example, the distances and angles for each lamp 165, 166 were set as follows: H5 = 87 mm, H6 = 82 mm, D5 = D6 = 130 mm, θ5 = 11.7 [deg], θ6 = 18.0 [deg].

Figure 14 is a view of the side of the tire 1 from the direction of the rotation axis. Figure 15 is a view of the tire 1 from below the lower surface 11D of the road plate 11. Figure 16 is a view of the side of the tire 1 from a direction perpendicular to the rotation axis.

Referring to Figures 14 to 16, lamps 167 and 168 are arranged on the underside 11D of the road plate 11. Lamps 167 and 168 are arranged at positions away from the tire 1 in the direction of tire travel. Lamps 167 and 168 are located on different sides of the tire 1.

As shown in Figures 14 and 15, lamps 167 and 168 preferably have a length in the tire width direction and emit light in a direction parallel to the tire travel direction. This improves measurement accuracy.

In Figure 14, H7 and H8 are the distances from the center of the light-emitting surface of each lamp 167 and 168 to the underside 11D of the road plate 11. In Figure 14, θ7 and θ8 are the inclination angles of each lamp 167 and 168, i.e., the angles of the light irradiation direction relative to the underside 11D of the road plate 11. In Figure 15, D7 and D8 are the distances in the tire traveling direction from the center of the light-emitting surface of each lamp 167 and 168 to the center of the tire 1.

In this example, the distances and angles for each lamp 167, 168 were set as follows: H7 = 92 mm, H8 = 94 mm, D7 = D8 = 400 mm, θ7 = θ8 = 2.1 [deg].

By continuously photographing the tire 1 contact patch under the above conditions, images of the tire 1 contact patch can be obtained. Images of the tire 1 contact patch are preferably taken every 3 degrees of tire 1 rotation angle, and more preferably every 1 degree. By taking images at intervals of this angle or less, the circumferential variation rate of the ACA can be obtained with high accuracy. In other words, the measurement angle is preferably 3 degrees or less, and more preferably 1 degree.

Figure 17 shows an example of an image obtained by photographing. Figure 17 shows an example of a general image, not the tread pattern of the tread portion 3 shown in Figure 2. The image shown in Figure 17 is the ACA. The ACA is the total area of the blocks that are in contact with the road surface. Based on the ACA shown in Figure 17, for example, the GCA shown in Figure 18 can be obtained.

The GCA is the total area enclosed by the outer ring line of the ACA when the grooves are filled. Figure 18 is a diagram showing an example of a GCA. For the ACA shown in Figure 17, for example, the GCA shown in Figure 18 can be obtained by performing the expansion process 80 times and then the contraction process 80 times.

Figure 19 is an explanatory diagram of the dilation process. Figure 20 is an explanatory diagram of the erosion process. As shown in Figure 19, the dilation process replaces a target pixel with a black pixel if there is even one black pixel surrounding the target pixel. In other words, the dilation process sets a white pixel as the center pixel, and if there is even one black pixel among the eight surrounding pixels (one pixel each in the upper left, upper, upper right, right, lower right, lower, lower left, and left directions closest to the center pixel), the center pixel is replaced with a black pixel. Conversely, the erosion process, for example, sets a target pixel as a black pixel, and if there is even one white pixel surrounding the target pixel, as shown in Figure 20, the target pixel is replaced with a white pixel. In other words, the erosion process sets a black pixel as the center pixel, and if there is even one white pixel among the eight surrounding pixels (one pixel each in the upper left, upper, upper right, right, lower right, lower, lower left, and left directions closest to the center pixel), the center pixel is replaced with a white pixel.

[Center contact width, center contact length, etc.]
Next, the center contact width and center contact length will be described. FIG. 21 is a diagram showing an example of a GCA. In FIG. 21, the maximum contact length Wr is the maximum length of the GCA in the tire forward direction. The position at half of the maximum contact length Wr, i.e., Wr/2, is the center position Wrc of the length in the tire forward direction. The center position Wrc of the length in the tire forward direction is the center position of one side in the tire circumferential direction of the smallest rectangle KK that surrounds the GCA. The length of the GCA in the tire width direction at the center position Wrc of the length of the GCA in the tire forward direction is the center contact width Wc. The center contact width Wc is the widthwise length of the GCA at the center position in the tire circumferential direction.

In addition, in Figure 21, the maximum contact patch width Wm is the maximum length of the GCA in the tire width direction. The position at half of the maximum contact patch width Wm, i.e., Wm/2, is the center position Wmc in the tire width direction. The center position Wmc in the tire width direction is the center position of one side in the tire width direction of the smallest rectangle KK that surrounds the GCA. The length of the GCA in the tire forward direction at the center position Wmc of the GCA's length in the tire width direction is the center contact patch length Wrr. The center contact patch length Wrr is the circumferential length of the GCA at the center position in the tire width direction.

[Another example of a tread portion]
Figure 22 is a diagram showing another example of the tread portion of a pneumatic tire. In Figure 22, the tread portion 3a has a configuration in which shoulder lug grooves 42 are added to the tread portion 3 of Figure 2. The shoulder lug grooves 42 are provided in the shoulder land portions 33. The shoulder lug grooves 42 are grooves that extend in the tire width direction. One end of the shoulder lug grooves 42 terminates within the shoulder land portions 33. The ratio of the pitch PT of the tread pattern of the tread portion 3a, i.e., the pitch ratio, is, for example, 1.38, 1.19, or 1.00.

The shoulder lug grooves 42 are provided at predetermined intervals in the tire circumferential direction. The secondary grooves 41 and shoulder lug grooves 42 are provided alternately in the tire circumferential direction. That is, in the shoulder land portion 33, one shoulder lug groove 42 is provided between two adjacent secondary grooves 41 in the tire circumferential direction. Also, in the shoulder land portion 33, one secondary groove 41 is provided between two adjacent shoulder lug grooves 42 in the tire circumferential direction. Because the secondary grooves 41 and shoulder lug grooves 42 are provided alternately in the tread portion 3a, the circumferential variation rate of the ACA can be reduced more than in the tread portion 3 of Figure 2.

The groove width of the shoulder lug grooves 42 is preferably 2 mm or less. If the groove width of the shoulder lug grooves 42 exceeds 2 mm, the impact of the shoulder lug grooves 42 on the ACA will be greater. As a result, the circumferential variation rate of the ACA will increase, and uneven wear resistance and wet braking performance will decrease.

Figure 23 is a diagram showing another example of a tread portion of a pneumatic tire. As shown in Figure 23, the tread portion 3b has a circumferential main groove 20 and a circumferential groove 43. The tread portion 3b does not have secondary grooves or lug grooves. In the tread portion 3b, the circumferential grooves 43 are provided in both the center land portion 31 and the shoulder land portion 33. The circumferential grooves 43 extend a predetermined length in the circumferential direction of the tire. The circumferential grooves 43 are not continuous in the circumferential direction of the tire, but are provided intermittently in the circumferential direction of the tire.

Even when tire 1 has tread portion 3b as shown in Figure 23, as explained with reference to Figure 3, it is preferable that the circumferential variation rate of ACA be 5.0% or less when the load rate is 70% of the load capacity by air pressure as specified in a predetermined standard. This improves resistance to uneven wear, i.e., heel-and-toe wear and polygonal wear (wear with a certain percentage of corners around the tire circumference).

[Circumferential Variation Rate of ACA, Center Contact Width, and Center Contact Length]
Here, the circumferential variation rate of the central contact width Wc is preferably equal to or less than the circumferential variation rate of the central contact length Wrr. The contribution of the contact width direction to uneven wear resistance and wet braking performance is significant. By suppressing the circumferential variation of the central contact width, uneven wear resistance and wet braking performance are improved.

If the circumferential variation rate of the center contact width Wc is greater than the circumferential variation rate of the center contact length Wrr, wet braking performance will deteriorate, which is undesirable. For example, the tread portion 3b in Figure 23 described above has a circumferential variation rate of the center contact width Wc that is greater than the circumferential variation rate of the center contact length Wrr, which is undesirable from the perspective of improving wet braking performance.

It is also possible to specify the circumferential variation rates of the two by using maximum contact width Wm instead of center contact width Wc and maximum contact length Wr instead of center contact length Wrr. That is, the circumferential variation rate of maximum contact width Wm can be set to a value equal to or less than the circumferential variation rate of maximum contact length Wr. However, specifying it in this way results in a mixture of tires with good uneven wear resistance and wet braking performance and tires with poor wet braking performance (e.g., Figure 23), making it impossible to narrow down the tires with good performance. For this reason, it is not appropriate to specify Tire 1 based on the circumferential variation rates of maximum contact width Wm and maximum contact length Wr.

It is preferable that the circumferential variation rate of the ACA for the tire 1 satisfies the following formula (2).
ACA peripheral fluctuation rate≦−3.4×A+7.38…(2)
In formula (2), "A" is the load factor. The load factor A is the ratio of the measured load to the load capacity by air pressure defined by the standard. The same applies to formulas (3) and (4) described below.

Figure 24 shows examples of ACA circumferential variation rates for various tires. The numerous points shown in Figure 24 represent the results of an analysis using static contact shape data from various tires at different circumferential positions measured at Yokohama Rubber Co., Ltd.'s testing facilities. The same applies to Figures 26 and 27, which will be described later. The line SS3 in Figure 24 corresponds to the ACA circumferential variation rate = -3.4 x A + 7.38. Tires whose ACA circumferential variation rate is equal to or less than line SS3 in Figure 24 are tires that satisfy equation (2).

When the circumferential variation rate of the ACA of tire 1 satisfies equation (2), uneven wear resistance (heel-and-toe wear and polygonal wear) and wet braking performance can be improved. As the load load factor A increases, uneven wear occurs even when the circumferential variation rate of the ACA is small. The slope of line SS3 is preferably negative -3.4. The intercept is automatically determined so that the circumferential variation rate of the ACA is 5.0% or less when the load capacity by air pressure specified in the specified standard is 70%.

Figure 25 is a table showing the tires analyzed. For each tire size, Figure 25 shows the product code, pattern, rim size, air pressure [kPa], load [kN], and number of circumferential locations analyzed. Note that all camber angles were set to zero.

26 is a diagram showing examples of the circumferential variation rate of the center contact length for various tires. For tire 1, it is preferable that the circumferential variation rate of the center contact length satisfies the following formula (3).
Circumferential variation rate of center contact length≦−3.3×A+8.0 (3)

The line SS4 in Figure 26 corresponds to the circumferential variation rate of the center contact length = -3.3 x A + 8.0. The slope is preferably negative -3.3. A tire whose circumferential variation rate of the center contact length is equal to or less than the line SS4 in Figure 26 satisfies equation (3).

27 is a diagram showing examples of the circumferential variation rate of the center contact width for various tires. For tire 1, it is preferable that the circumferential variation rate of the center contact width satisfies the following formula (4).
Circumferential variation rate of center contact width≦−1.9×A+3.9… (4)

The line SS5 in Figure 27 corresponds to the circumferential variation rate of the center contact width = -1.9 x A + 3.9. The slope is preferably negative -1.9. A tire whose circumferential variation rate of the center contact width is equal to or less than the line SS5 in Figure 27 satisfies equation (4).

By satisfying formulas (3) and (4), uneven wear resistance can be further improved. However, as the load factor A increases, uneven wear will occur even if the circumferential variation rates of the center contact length and center contact width are small.

[Example]
In this example, performance tests were conducted on uneven wear resistance and wet braking performance for several types of tires under different conditions (see Table 1). In this performance test, new test tires were mounted on specified wheels and mounted on a test vehicle (sedan) with an engine displacement of 3 L, and the air pressure was set to 250 kPa.

To evaluate uneven wear resistance, the tire was fitted to a test vehicle and driven 10,000 km, after which a laser displacement meter was used to obtain a shape profile of the tire's tread, which was then compared with the shape profile of the tread of a new tire. The difference in the tire's radial length in the tread was measured as the amount of wear, and the average amount of wear across the entire tread was calculated. The average amount of wear was determined by setting the conventional example as the standard (100), dividing the average amount of wear in the conventional example by the average amount of wear in the example, and multiplying the result by 100 to obtain an index. The higher the index value, the better the uneven wear resistance.

For wet braking performance, braking distance was measured at a speed of 100 km/h on a wet road surface with 1 mm of water depth. The braking distance for the conventional example was set as the standard (100), and the braking distance for the conventional example was divided by the braking distance for the example tire and multiplied by 100 to obtain an index. The higher the index value, the better the wet braking performance.

All of the tires in Examples 1 to 7 in Table 1 have sub-grooves in the tread with a groove width of 2 mm or less. The conventional tire in Table 1 has sub-grooves in the tread with a groove width of 2.5 mm.

As shown in the test results in Table 1, the tires of each example demonstrate excellent resistance to uneven wear and wet braking performance.

1 Pneumatic tire 2 Tread rubber 3 Tread portion 5 Drive device 6 Sidewall portion 7 Motor control device 10 Bead portion 11 Road surface plate 12 Bead filler 13 Carcass layer 14 Belt layer 15 Camera 16 Illumination lamp 17 Rim cushion rubber 20 Circumferential main groove 21 Input unit 22 Display unit 23 Processing device 23a Processing unit 31 Center land portion 33 Shoulder land portion 35 Memory unit 41 Sub-groove 42 Shoulder lug groove 43 Circumferential groove 60 Motor 61 Ground contact surface 100 Tire ground contact shape analysis device 110 Bead cores 141, 142 Cross belt 143 Belt cover 150 Inner liner 160 Sidewall rubber 200 Tire testing machine 201 Tire ground contact surface analysis device 231 Road surface drive unit 232 Ground contact characteristic acquisition unit 233 Ground contact characteristic variation calculation unit 233a Circumferential average value calculation unit 233b Circumferential fluctuation rate calculation unit 300 Support device CL Tire equatorial plane

Claims (9)

The tire comprises a carcass layer that forms a framework of the tire, a belt layer that is disposed on an outer periphery of the carcass layer, and a tread portion that is provided on the outer side of the belt layer,
The tread portion is
At least two circumferential grooves extending in the tire circumferential direction;
a shoulder land portion extending in the tire circumferential direction and including a tire ground contact edge;
A sub-groove is a groove that does not cross the tire equatorial plane, extends in the tire width direction, and does not extend to the outer side of the shoulder land portion in the tire width direction, excluding the circumferential groove portion;
Equipped with
The auxiliary groove is inclined with respect to the tire circumferential direction over its entire length,
the sub-groove is arranged to intersect with the circumferential main groove,
The auxiliary groove has a short portion on an outer side in the tire width direction and a long portion on an inner side in the tire width direction, with the circumferential main groove as a boundary,
a groove center line of the short portion and a groove center line of the long portion are on the same straight line,
the inclination direction of the sub-groove with respect to the tire circumferential direction is the same on both sides of the circumferential main groove that intersects with the sub-groove,
A pneumatic tire in which, when the load rate is set to 70% of the load capacity by air pressure as specified in a predetermined standard and the road plate is moved while the tire is pressed against the road plate, the circumferential variation rate of the actual contact area image is 5.0% or less. When the length along the tire circumferential direction passing through the center of the total contact area obtained from the tire contact shape is defined as the center contact length, and the length along the tire width direction passing through the center of the total contact area is defined as the center contact width,
The pneumatic tire according to claim 1 , wherein the circumferential variation rate of the center contact length is equal to or greater than the circumferential variation rate of the center contact width. The pneumatic tire according to claim 2 , wherein the circumferential variation rate of the center contact length and the circumferential variation rate of the center contact width are defined by the following formulas:
Circumferential variation rate of center contact length≦-3.3×A+8.0
Circumferential variation rate of center contact width≦-1.9×A+3.9
Load factor A = (measured load) / (load capacity by air pressure specified in the standard) A pneumatic tire according to any one of claims 1 to 3, wherein the auxiliary grooves, each having a groove width of 2 mm or less, are provided at predetermined intervals around the tire in land portions adjacent to the circumferential grooves. When a groove extending in the tire width direction and extending to the outer side of the shoulder land portion in the tire width direction is defined as a shoulder lug groove,
The pneumatic tire according to claim 4, wherein the width of the shoulder lug groove is 2 mm or less. The pneumatic tire of claim 5, wherein the secondary grooves and the shoulder lug grooves are arranged alternately in the tire circumferential direction. The auxiliary groove extends across the circumferential groove,
The pneumatic tire according to any one of claims 1 to 6, wherein an angle of the extension direction of the auxiliary groove with respect to the tire width direction is equal to or greater than 0 degrees and equal to or less than 45 degrees. A pneumatic tire according to any one of claims 4 to 7, wherein the tread portion has a tread pattern that repeats at a predetermined pitch, and the pitch ratio of the predetermined pitch is 1.38, 1.19, or 1.00. The pneumatic tire according to claim 1 , wherein the circumferential variation rate of the actual contact area image is defined by the following formula:
Circumferential fluctuation rate of actual contact area image≦−3.4×A+7.38
Load factor A = (measured load) / (load capacity by air pressure specified in the standard)